1. Field of the Invention
The present invention pertains to communications systems, and particularly to communications systems which employ ATM technology.
2. Related Art and Other Considerations
Asynchronous Transfer Mode (ATM) is becoming increasingly used in communication networks. ATM is a packet-oriented transfer mode which uses asynchronous time division multiplexing techniques. Packets are called cells and have a fixed size.
As shown in FIG. 1, an ATM cell consists of 53 octets, five of which form a header and forty eight of which constitute a xe2x80x9cpayloadxe2x80x9d or information portion of the cell. The header of the ATM cell includes two quantities which are used to identify a connection in an ATM network over which the cell is to travel, particularly the VPI (Virtual Path Identifier) and VCI (Virtual Channel Identifier). In general, the virtual path is a principal path defined between two switching nodes of the network; the virtual channel is one specific connection on the respective principal path.
Between termination points of an ATM network a plurality of nodes are typically situated, such as switching nodes having ports which are connected together by physical transmission paths or links. The switching nodes each typically have several functional parts, a primary of which is a switch core. The switch core essentially functions like a cross-connect between ports of the switch. Paths internal to the switch core are selectively controlled so that particular ports of the switch are connected together to allow a cells ultimately to travel from an ingress side of the switch to an egress side of the switch.
A protocol reference model has been developed for illustrating layering of ATM. The protocol reference model layers include (from lower to higher layers) a physical layer (including both a physical medium sublayer and a transmission convergence sublayer), an ATM layer, and an ATM adaptation layer (AAL), and higher layers. The basic purpose of the AAL layer is to isolate the higher layers from specific characteristics of the ATM layer by mapping the higher-layer protocol data units (PDU) into the information field of the ATM cell and vise versa. There are several differing AAL types or categories, including AAL0, AAL1, AAL2, AAL3/4, and AAL5.
AAL2 is a standard defined by ITU recommendation 1.363.2. An AAL2 packet is shown in FIG. 2 as comprising a three octet packet header, as well as a packet payload. The AAL2 packet header includes an eight bit channel identifier (CID), a six bit length indicator (LI), a five bit User-to-User indicator (UUI), and five bits of header error control (HEC). The AAL2 packet payload, which carries user data, can vary from one to forty-five octets
FIG. 3 shows how plural AAL2 packets can be inserted into a standard ATM cell. In particular, FIG. 3 shows a first ATM cell 201 and a second ATM cell 202. Each ATM cell 20 has a header 22 (e.g., cell 201 has header 221 and cell 202 has header 222). The payload of the ATM cells 20 begin with a start field 24 (e.g., cell 201 has start field 241 and cell 202 has start field 242). After each start field 241 the ATM cell payload contains AAL2 packets. For example, the payload of ATM cell 201 contains AAL2 packets 261 and 262 in their entirety, as well as a portion of AAL2 packet 263. The payload of cell 202 contains the rest of AAL2 packet 263, and AAL2 packets 264 and 265 in their entirety. In addition, the payload of cell 202 has padding 28.
The start field 24, shown in FIG. 3A, facilitates one AAL2 packet bridging two ATM cells. Start field 24 includes a six bit offset field (OSF), a one bit sequence number (SN), and one parity bit (P). The six bit offset field (OSF) contains a value, represented by offset displacement 29 in FIG. 3, indicative of the octet in the payload whereat the first full AAL2 packet begins. For ATM cell 221, the value of the offset field (OSF) is one, since AAL2 packet starts just after start field 241. For ATM cell 222 the value of the offset field (OSF) is the sum of one (in view of start field 241) and the number of octets of AAL2 packet 263 protruding into cell 222.
AAL2 advantageously allows multiplexing of data from many users within a single ATM VCC. In such multiplexing scheme, each user""s data is carried in a separate AAL2 packet, but AAL2 packets of differing users are carried in the same ATM cells or cells borne on the same ATM VC. Thus, assuming each user has a different channel identifier (CID) value, as many as 248 user channels can be multiplexed onto one ATM VC. AAL2 thus allows more efficient utilization of low speed links than standard ATM while still maintaining low delay properties.
A problem with using AAL2 arises when terminating AAL2 channels at different nodes or different addresses in the same node. Since the individual AAL2 channels may be multiplexed into one ATM-VCC, it is not possible to direct the individual AAL2 channels (e.g., the AAL2 packets on which the channel data is carried) to different destinations using conventional ATM switches.
One approach for switching AAL2 packets is set forth by Mauger and Rosenberg, xe2x80x9cQoS Guarantees for Multimedia Services on TDMA-Based Satellite Networkxe2x80x9d, IEEE Communications Magazine July 1997). In that approach, fixed-cell ATM switches work in conjunction with separate variable-cell ATM switches for handling AAL2 packets.
A communications network has ATM cells with AAL2 protocol packets carried on an ATM link between two nodes of the network. Using the AAL2 protocol packets, many user channels are multiplexed onto one ATM VC between the two nodes. In one of the two nodes designated as a control node, user channels are terminated by mapping AAL2 packets of the user channels into ATM cells utilizing an AAL protocol different than AAL2. A new AAL protocol, termed AAL2 prime, requires that AAL2 packets carried in the ATM cell payload be whole packets and that the ATM payload not have an AAL2-type start field. Preferably, in the AAL2 prime protocol only one whole AAL2 packet is carried per ATM cell payload.
Termination of the AAL2 user channels at the control node preferably occurs at a pooled and centralized resource, herein termed a cell handling unit or AAL termination unit (ATU). Thus, the cell handling unit can terminate AAL2 user channels from plural nodes controlled by the control node. The cell handling unit performs, among other things, both demultiplexing and multiplexing operations. In the demultiplexing operation, the cell handling unit uses ATM cells having AAL2 packets mapped onto ATM-VCC according to AAL2 to form ATM cells containing AAL2 packets mapped according to the AAL2 prime protocol. In the multiplexing operation, ATM cells containing AAL2 packets mapped according to the AAL2 prime protocol are received and the AAL2 packets are moved to outgoing ATM cells and mapped according to AAL2.
The provision of the cell handling unit and the AAL2 prime protocol advantageously allows standard ATM equipment, e.g., ATM switch cores, to be employed. By using the AAL2 prime protocol, it is possible to route each individual AAL2 channel to its destination by routing the carrying ATM-VCC to that destination. ATM cells containing AAL2 packets which are received at a control node can be routed through the control node to the cell handling unit. At the cell handling unit the ATM cells containing AAL2 packets are demultiplexed to form ATM cells having AAL2 prime packets. The AAL2 prime packets can then be routed through the ATM switch to other elements of the control node which cannot handle ATM cells with AAL2 packets.
In addition to the multiplexing and demultiplexing functions, the cell handling unit performs queuing of ATM cells which are outbound from the node. Further, in one mode of the invention, ATM cells having the AAL2 protocol can be transmitted from the control node to other nodes, e.g., nodes superior to the control node. In another mode of the invention, when the link between the control node and the node superior is delay sensitive, the cell handling unit transforms the ATM cells which it receives with AAL2 prime protocol to ATM cells with AAL2 packets for transmission on the delay sensitive link. In yet another mode, ATM cells with AAL2 prime protocol can be sent on the link to the superior node, e.g. until it is sensed that delay on the link exceeds a predetermined threshold.
A mobile telecommunications network is provided as an example illustrated embodiment. In the telecommunications network a base station control node (e.g., base station controller) controls plural base stations. A xe2x80x9csuper-Axe2x80x9d interface between the base stations and the base station controller node carries the ATM cells with AAL2 packets. At the base station controller node the cell handling unit of the base station controller transforms the ATM cells with AAL2 packets into ATM cells having the AAL2 prime protocol. The ATM cells resulting from the transformation effected by the base station controller can then be forwarded to other units of the base station controller, such as a diversity handover (DHO) unit. An xe2x80x9cAxe2x80x9d interface exists between the base station controller and a node superior thereto, e.g., a mobile switching center (MSC).
In one telecommunications embodiment, the ATM cells with the AAL2 prime protocol are transmitted over the A interface between the base station controller and the mobile switching center (MSC). In this embodiment, the ATM cells with AAL2 prime protocol destined for transmission to the mobile switching center (MSC) can be returned to cell handling unit for queuing prior to transmission to the mobile switching center (MSC). In another telecommunications embodiment, ATM cells with the AAL2 protocol are transmitted over the A interface. In this second telecommunications embodiment, the ATM cells with the AAL2 prime protocol must be routed back to the cell handling unit, so that the cell handling unit can transform these cells into ATM cells with AAL2 packets and properly queued for transmission to the mobile switching center (MSC).